Method for determining crystallographic orientation of single crystals



March 1961 P. R. PENNINGTON ET AL 2,973,687

METHOD FOR DETERMINING CRYSTALLOGRAPHIC ORIENTATION OF SINGLE CRYSTALS Filed April 2, 1958 2 Sheets-Sheet 2 Percent coac enimfloi K0 H Philip R. Pennington, John D. Turner, INVENTORS.

ATTORNEX March 1961 P. R. PENNINGTON ETAL 2,973,687

METHOD FOR DETERMINING CRYSTALLOGRAPHIC ORIENTATION 0F SINGLE CRYSTALS Filed April 2, 1958 2 Sheets-Sheet 1 Philip R. Pennington, John D. Turner,

INVENTORS.

A T TORNE' X Unite 1 METHOD FOR DETERMINING CRYSTALLO- G gi-no ORIENTATION F SINGLE CRYS T Filed Apr. 2, 1958, Ser. No. 72S,96j0 Q 4 Claims. (Cl. 88-14) This invention relates to a method for determining the disposition of a particular crystallographic plane in a single crystalline body and for orienting the body with respect to this plane. More particularly, the invention relates to a method for determining the disposition of the (111) crystallographic plane in the crystal lattice structure of a body of single crystalline silicon and for orienting the body with respect to this plane.

By now it is well known to employ semiconductive single crystalline bodies for transistor devices and the like. In order to obtain optimum device performance it is desirable to have the surfaces of the single crystalline bodies substantially parallel to the (111) plane of the crystal lattice ofthe body, and to form rectifying junc tions on planes coincident with or substantially parallel to the (111) plane. In general, these bodies are obtained by growing single crystals in the form of an elongated ingotin the (111) direction and then slicing the ingot pcrpendicularlyto the growth direction and thus on the (111) plane. It will be appreciated that prior to this slicing operation, the precise disposition of the (111) plane in the ingot must be determined so that the ingot may be properly oriented with respect to the slicing equipment or saw. That is, the ingot must be so mounted in the saw, for example, that its (111) plane is parallel to thecutting plane of the slicing blade.

,Heretofore, single crystalline ingots have been oriented with respect to the (111) plane by meansv of techniques utilizing the diffraction of X-rays by the crystal lattice. Such X-ray, techniques have required expensive equipment and have presented radiation hazard problems which are, of course, undesirable in a production process.

It is therefore an object of the instant invention to provide an improved method for determining the disposition of a particular crystallographic plane in the crystal lattice of a single crystalline body andfor orienting the body with respect to this crystallographic plane.

A further object of the invention is to provide an improved method for determining the disposition of the (111) plane in the crystal lattice of a single crystalline body of silicon and for orienting the body with respect to the (111) plane.

7 These and other objects and advantages of the invention are accomplished by first preferentially developing the particular crystal plane with which it is desired to orient the single crystalline body. A beam of light is then directed onto the developed crystal plane and a reflection from the plane is utilized to orient the crystal.

The development of the particular crystalline plane is achieved by etching a surface of the crystalline body. The direction of reflection from the crystal plane, which acts as a little mirror, depends upon the disposition of the plane withrespect to the incident light beam. Thus by making the direction of the reflection from the crystal plane coincide with the direction of reflection from an optical reference plane, the direction of the optical reference plane will be the same as the direction of the particular crystal plane in the crystalline body. It is rates Patent. 0

ice

desirable that the light reflected from the etch-developed crystal plane should form a substantially well-defined optical image so as to facilitate the step of making the direction of reflection from the crystal plane coincide with 'the'directio'n of reflection from the reference plane. In the case of a single crystal of silicon, such a light figure in the form of a somewhat triangular ring with a small dot in the center thereof may be obtained by etching the single crystalline surface with a sodium hydroxide solution. With such an etching solution, the (111) crystal plane may be developed in preference to other crystallographic planes since the rate of etching is slowest perpendicularly to the (111) plane by a sodium hydroxide solution in comparison with the rate of etching perpendicularly to other crystallographic planes. Hence, at the end of a fixed period of etching, only the (111) crystalline plane will have been developed.

The invention will be described in greater detail by reference to the drawings in which:

Fig. 1 is a schematic view of an optical system for obtaining reflections from a crystal plane of an etched crystal surface; and

Figs. 2-10 show a series of silicon crystal surfaces etched with different concentrations of sodium hydroxide for different periods of time.

The first step for determining the disposition of the (111) crystal plane in a single crystal of silicon is to etch a surface of the crystal so as to preferentially develop the (111) plane and to permit the formation of a well-defined optical image from the etch-developed plane. As will be demonstrated hereinafter, the size (diameter) of the light figures obtained according to the instant invention depends primarily on the concentration of the etching solution. On the other hand, the sharpness of the images of light figures is determined primarily by the duration of etching. It will also be appreciated that the concentration of the etching solution also determines the rate and hence the time of etching. As mentioned heretofore, the preferred light figure comprises a circle with a small dot in the center thereof. The size of the dot decreases with etching time and can become non-observable if etching is continued for too longa time. In general, satisfactory light figures can be obtained by etching with boiling sodium hydroxide solutions having a concentration of from 35 %45 for about 1 /2 to 2 or 3 minutes. Even if the dot is not readily observable, the circle may be employed, according to the invention, to determine the position the dot would have ifobservahle (substantially at the center of the circle).

Another reason for preferring a solution having a range of 35%-45% in concentration lies in the fact that solutions of lower concentration (i.e., 20%) will not result in satisfactory light figures if the original crystal surface is badly misoriented (i.e., more than about 12) from the desired 111) crystal plane. Stated differently, the more nearly parallel or coincident the surface of the crystal is with respect to the *(111) plane, the less concentrated the etching solution need be in order to obtain the desired light figures. Since, however, solutions having a concentration range of from 35% to 45% will result in completely acceptable light figures for surfaces misoriented from the (111) plane by as much as 12, it is preferred. This is particularly true in view of the fact that the surfaces of most of the crystals produced by semiconductor manufacturing techniques are oriented obtained with highly concentrated solutions (i.e., more than about 50%) are less observable and reliance must be had upon the circle formed in order to determine the dot position.

Example I A body of single crystalline silicon is etched with a boiling aqueous solution of sodium hydroxide for two minutes. The concentration of the solution is 42.5%. A reflected optical image is formed by directing a beam of light onto the etched surface. This image is a substantially well-formed circle with a small white dot in the center thereof. The white dot is a reflection from the (111) crystal plane. The reflected optical image appears substantially as shown in Fig. 6.

Example II A surface of a single crystal of silicon is etched with a boiling aqueous solution of sodium hydroxide having a concentration of 35%. The etching time is about 3 minutes. Again the configuration of the optical image formed by directing a light beam onto the etched surface is substantially as shown in Fig. 6 except that the diameter of the dot is not as great as in the case of the light figure obtained in Example I.

Example III The surface of a single crystal of silicon is etched with a boiling sodium hydroxide solution having a concentration of 45% for about 1 /2 minutes. The configuration of the optical image obtained by directing a light beam on the etched surface is substantially the same as shown in Fig. 6 except that the diameter of the dot is greater than that of the dot obtained in Example I.

Referring to Figs. 2-10, it will be noted that acceptable light figures may be obtained by etching with sodium hydroxide solutions of almost any concentration up to and including 50%. Figs. 2, 3 and 4 show the reflected optical images obtained from silicon crystals etched with a boiling 30% sodium hydroxide solution for 1, 2 and 5 /2 minutes, respectively. It will be observed that the light figures obtained from these etched surfaces are sufliciently well-formed to permit observation of the dot itself (Fig. 3) or to determine its position by means of the somewhat triangularly-shaped figures of Figs. 2 and 4. However, should the surface of the crystal be badly misoriented, the light figures obtained with a 30% etching solution will tend to make orientation of the single crystalline body with respect to the (111) plane somewhat more diflicult.

The optical images shown in Figs. 5, 6 and 7 are obtained from the surface of single crystalline silicon bodies etched with a 40% sodium hydroxide solution (boiling) for 1, 2 and 5 /1 minutes, respectively. These images exhibit either a readily'observable dot (Fig. 6) or may be employed to accurately determine the dot position (Figs. 5 and 7). As noted from the examples given previously, Fig. 6 shows the configuration of the optical image obtained from an etched silicon surface employing either a 35% sodium hydroxide solution for 3 minutes or a 42 /2% sodium hydroxide solution for 2 minutes, or a 45 sodium hydroxide solution for 1 /2 minutes.

Figs. 8, 9 and 10 show the light figures obtained from etch-developed planes in single crystalline silicon bodies by etching with 50% sodium hydroxide solution (boiling) for l, 2, and 5 /2 minutes. It will be noted that while substantially well-formed figures are obtained and may be employed for the purposes of the invention, the development of the light dot in the center of the figures is not as satisfactory in comparison with the light figure shown in Fig. 6, for example.

An optical image, such as shown in Fig. 6, consisting of a circle with a small dot in the center thereof, indicates that the etched surface is reflecting light from a nearly perfectly developed (111) plane of the surface of the silicon crystal body. The dot itself is a reflection from the (111) crystal plane. Sodium hydroxide solutions are preferred for developing the 111) crystal plane since this etchant preferentially develops the (111) plane in comparison with other crystallographic planes.

Referring now to Fig. 1, after the surface of a single crystalline body 2 of silicon has been etched to obtain the optimum optical image from the (111) crystal plane, it may be mounted in a universally movable jig (not shown) to which a mirror 4 or other optically reflecting surface may be adjustably secured. The jig permits separate and independent positioning and locking of the mirror 4 and the crystalline body 2. A beam of light 6 from a point source 8 is directed by means of the projection lens 10 onto the etched surface of the silicon body so that the reflected optical image from the etched surfaceis focused onto a screen 12 and centered thereon by means of cross-hairs provided on the screen. The crystalline. body, is then locked into position on the jig which may then be moved so as to permit the light beam 6 to fall upon the surface of the mirror 4. The position of the mirror 4 is then adjusted so as to center the optical image of the light source 8 on the cross-hairs of the screen 12. It will then be apparent that the direction of reflection from the mirror 4 coincides with the direction of reflection from the etch-developed crystal plane, and hence the plane of the mirror is parallel to the crystal plane. The mirror may then be locked into position on the jig and thus serve as an indication of the direction of the (111) crystallographic plane in the crystal body 2. Thereafter, the crystal, secured to the jig, may be moved to a saw, for example, and sliced on the (111) crystal plane by making the plane of the saw blade parallel to the referencing mirror 4. In practice, it was found that single crystalline bodies of silicon can be consistently oriented and adjusted to within plus or minus 5 minutes of correct orientation with respect to the (111) crystallographic plane, as determined by the X-ray diffraction technique.

It will be appreciated that variations in this technique will occur to those skilled in the art which variations will be within the scope of the invention. For example, the mirror 4 may be first oriented so as to be parallel with the effective cutting plane of the slicing equipment. This may be accomplished by cutting a body of any rigid material on the particular saw involved and then positioning and locking the mirror in the jig so that it is parallel to the cut surface of this body. The light beam 6 is now reflected from the mirror 4 and focused onto the screen 12. The screen 12 is then adjusted and locked so that the focused image of the light source 8 is accurately positioned onto the cross-hairs thereof. When the light figure from an etch-developed (111) crystal plane on the surface of a crystal body 2 is focused on the crosshairs of the screen 12, the direction of reflection from this (111) plane will coincide with the direction of reflection from the mirror 4. Therefore, the (111) plane in the crystal body 2 will be parallel to the plane of the mirror 4 and hence parallel to the effective cutting plane of the saw.

In another embodiment of the invention which permits the direction of reflection from the mirror 4 and the direction of reflection from an etch-developed (111) crystal plane to be made much more precisely coincident, a photocell is substituted for the screen 12. A shield with a small aperture therein is positioned between the photocell and the reflecting surface. The aperture is small enough so that substantially all of the reflected light passes therethrough only when the reflected light beam is substantially centered thereon. By observing the strength of the signal generated by the photocell in accordance with the light falling thereon, the reflecting surface can be accurately positioned. For example, the maximum voltage will be obtained only when the maximum amount of light falls upon the photocell and this situation will obtain only when the reflected light beam is substantially centered on the aperture in the shield.

There thus has been described a novel method for determining the direction of the (111) crystallographic plane in single crystalline bodies of silicon which method is relatively rapid and inexpensive in contrast to the methods of the prior art.

What is claimed is:

l. The method of orienting a single crystalline body of silicon with respect to the (11]) crystallographic plane 7 therein comprising the steps of etching a surface of said body with a boiling solution of sodium hydroxide having a concentration of from about 35% to about 45% for up to about three minutes to thereby develop said (111) crystallographic plane in preference to other crystallographic planes; illuminating said etch-developed (111) crystallographic plane whereby the reflected illumination from said plane forms an optical image having substantially the configuration of a circle with a small dot in the center thereof; and referencing the plane of said body parallel to the plane of reflection of said image.

2. The method according to claim 1 wherein said (111) crystallographic plane is etched for about two minutes with a boiling aqueous solution of sodium hydroxide having a concentration of about 42Vz%.

of silicon with respect to the (111) crystallographic plane therein comprising the steps of: etching a surface of said body with a boiling solution of sodium hydroxide for up to three minutes, said solution having a concentration of from about to projecting a light beam in a given direction onto an optical reference surface; determining the direction of reflection of an image from said optical reference surface; projecting said light beam in said given direction onto said etch developed (111) crystal plane; and utilizing the optical image reflected from said etch-developed 111) plane to make the direction of reflection from said etch-developed (111) plane coincide with the direction of reflection of said image from said optical reference surface.

References Qited in the file of this patent Hancock, R. D. et al.: Simplified Light Reflection Technique for Orientation of Germanium and Silicon Crystals, pages 1082, 1083 in Review of Scientific Instruments, December 1956. 

